This invention relates generally to ultrasonic inspection of shrunk-on steam turbine disks or hubs and more particularly to the ultrasonic inspection of the disk bore and keyway area to detect stress corrosion cracking.
Turbines are used to generate rotary mechanical power from the energy in a working fluid. The working fluid energy, originally in the form of pressure energy, is converted to velocity energy by passing through a system of blades in the turbine. Changes in the magnitude and direction of the velocity energy are made to cause tangential forces on the blades to produce mechanical rotation of the turbine rotor. The rotating turbine rotor may be coupled to a generator rotor and stator to produce electricity.
Steam turbines are used to convert thermal energy into usable work and are typically used to drive alternating current electric generators. A large number of xe2x80x9cbuilt-upxe2x80x9d low pressure, steam turbine rotors are operating in the U.S. electric utility industry. FIG. 1 and FIG. 2 illustrate an example of a simplified rotor assembly. A built-up rotor 10 generally consists of a large central rotor shaft 12 with individual disks or blade hubs 14 fitted to the shaft. Individual disks carry one or more stages of blades 16, or buckets, that capture the incoming steam and transform it to torsional energy. The disks have a central bore hole 20 that matches a specific machined step on the central shaft as illustrated in FIG. 2. The disks are typically shrunk onto the shaft and subsequently restrained from torsional movement, relative to the shaft, by use of a key 22. A machined slot, or keyway 24, cut into the disk and shaft accommodates the key. Utility operating experience with built-up rotors using shrunk-on disks has demonstrated that the area around the central bore hole 20 of the disk or hub 14, and especially the area around the keyway 24 on the rotor hub 14, are subject to stress corrosion cracking (SCC) that can ultimately lead to catastrophic failure.
A variety of ultrasonic inspection techniques are used to examine various components that make up a steam turbine rotor. One method using phased array ultrasonic sensors to inspect turbine blade attachments is described in U.S. Pat. No. 6,082,198 the text of which is hereby incorporated by reference. Ultrasonic techniques involve applying high frequency sound waves to a structure of interest. When the sound waves interact with an object that has a significant difference in acoustic impedance (the product of density and acoustic velocity) from that of the propagation medium, a portion of the sound is either reflected or diffracted back to the source from which the sound originated. Detection and quantification of the returned sound pattern is used to determine the presence and characteristics of the reflecting medium.
Ultrasonic inspections of the disk bore 20 and keyway area 24 have historically been performed to detect SCC as illustrated in FIGS. 3A and 3B. The keyway area is the highest stress point on the disk bore, so substantial effort is invested in detecting and sizing cracks in this area. Current state of the art techniques for inspection of the disk bore area use a series of individual, conventional, single angle, ultrasonic probes 26. These probes 26 are placed on the disk face and angled to transmit toward the keyway area 24. Reflections and diffractions from the area and the disk edges are recorded automatically or manually and then analyzed in relation to the known geometry of the keyway area. These prior art methods are most effective if used in conjunction with detailed existing data on the geometry of the keyway area. Additionally, because of the need for numerous probes operating at fixed angles, the process is relatively slow and can only be performed while the turbine is out of service.
Accordingly, it is a general object of the present invention to provide a method for using advanced ultrasonic techniques to inspect the disk bore and keyway area of a shrunk-on steam turbine disk.
More specifically, it is an object of the current invention to provide a method employing linear phased array probe technology to ultrasonically inspect the disk bore and keyway area of a shrunk-on steam turbine disk.
Alternatively, it is an object of the present invention to provide a method for ultrasonically inspecting the disk bore and keyway area of a shrunk-on steam turbine disk that employs time of flight diffraction (TOFD) techniques.